In smoke detectors of the type that utilize a photo-responsive device viewing smoke particles in a light beam, it is desirable that the detector respond only to smoke of a predetermined concentration and not to lower concentrations, to provide a tolerance in response to avoid false alarms from variations in response due to temperature, aging of the photo-responsive device, changes in line voltage, and changes in intensity of the light source.
Various means have been provided for this purpose. In most cases, a second photo-responsive device is placed electrically in series with the photo-responsive device viewing the light beam, and is physically positioned to view the light beam directly, with means provided for adjusting the amount of light received by the photo-responsive device from the light source. However, such an arrangement does not prevent change of the alarm point with line voltage and with aging of the light source since the light output varies with line voltage and some light sources, such as light emitting diodes, vary greatly in light output with temperature changes.
Another defect of such detectors utilizing the reflected light principle is their relatively poor response to black smoke. This type of smoke has low reflectivity, and therefore does not reflect sufficient light onto the detector cell to cause the alarm to be activated.